Hitherto, for years there has been known a method for producing silica sol through neutralization or ion-exchange of water glass serving as a raw material. Also, it has been known that silica micropowder can be produced through pyrolysis of silicon tetrachloride. An alternative known method for producing a high-purity silica sol is based on hydrolysis of a silicon alkoxide in an alcoholic aqueous solution in the presence of a basic catalyst. In one reported method, a 0.28 mol/L tetraethyl silicate is added to an alcoholic solution containing several mol/L-order ammonia and several mol/L to 15 mol/L water, and the mixture is hydrolyzed, to thereby obtain silica particles having a diameter of 50 to 900 nm (see, for example, Non-Patent Document 1).
Another known method for producing silica sol includes hydrolysis of a silicon alkoxide in the presence of ammonia serving as a hydrolysis catalyst. An alternative method is based on hydrolysis of a silicon alkoxide in the presence of an organic base compound serving as a hydrolysis catalyst. In one exemplary disclosed method, silica sol is produced through hydrolysis of tetraethyl silicate in the presence of a catalyst such as tetramethylammonium hydroxide (see, for example, Patent Document 1). Another method for producing neutral colloidal silica is disclosed, which method includes hydrolysis of tetramethyl silicate in the presence of a catalyst containing a quaternary ammonium salt, an aminoalcohol, a morpholine, or a piperazine (see, for example, Patent Document 2).
Another disclosed silica sol production method includes adding ammonia to a silica sol containing silica particles formed under specific conditions, whereby the sol is concentrated under control of pH (see, for example, Patent Document 3). A similar known method for producing a colloidal silica composition includes adding tetraethylammonium hydroxide to a colloidal silica composition containing silica particles formed under specific conditions, to thereby adjust the hydrogen ion concentration, and then concentrating the colloidal silica (see, for example, Patent Document 4).